Birds are so much smaller than ourselves that we might be tempted to think of them as all being roughly the same size.
That would be a mistake, even when it comes to species in the same family. When we see them together like this, it’s obvious that Feral Pigeons (Columba livia) are much bigger than Peaceful Doves (Geopelia placida or G. striata, depending on your source). Birds In Backyards gives us the numbers, 34 cm vs 22 cm long, and weighing 300 gm vs 54 gm respectively.
Both species are members of the Columbidae family, doves and pigeons. The Peaceful Dove is one of our two smallest Columbidae (the Diamond Dove, much less common in coastal areas, is the other). The Feral Pigeon is by no means the largest; the Torresian Imperial-Pigeon is 39 – 44 cm, and and there’s a fruit dove in Arnhem Land which reaches 50 cm.
Most of us know that the Feral Pigeon, more politely referred to as the Rock Dove, is an introduced species. It’s unlike most introduced birds, however, in that it arrived as a domesticated species and escaped into the wild, making it more like our feral pigs and cats than (e.g.) sparrows. If you want to know more – and it’s a species with a long and varied human history – Wikipedia’s article is excellent and this Cornell Lab page has some “Cool Facts” which make it worth a visit.